The present invention relates to the preparation of bulk materials which may range from being completely amorphous to completely crystalline. The bulk materials are produced by a rapid solidification process, specifically liquid dynamic compaction, in which the different products are produced by varying the operating conditions. Generally the process entails delivering a stream of a molten metal alloy into an inert gas atmosphere and atomizing it with an inert gas by means of one or more ultrasonic inert gas jets. The atomized alloy droplets impact a high heat capacity substrate, preferably liquid cooled, to form "splats" which build up upon themselves to form the desired bulk rapidly solidified material. The resultant bulk materials, be they amorphous or crystalline, generally contain little or no oxygen greater than that in the initial starting materials used to form the molten metal alloy. The term "bulk" is used herein to mean a product having a thickness of at least 250 microns, preferably at least about 1 mm, and more preferably at least about 3 mm. The bulk materials are thus directly prepared, i.e. without first crushing or comminuting the deposited material to form a powder and then reconsolidating that powder into a shaped bulk product. As a result, the initial microstructure of the deposited material, be it amorphous or crystalline, can be maintained in the final product. Alternatively, when some bulk amorphous materials are produced, they can be heat treated in a controllable manner to alter their structures and to convert them to bulk permanent magnets having superior magnetic properties.
Over the past few years, iron-neodymium-boron (Fe-Nd-B) alloys have attracted growing interest as high performance permanent magnets. High coercivities were reported for Fe-Nd films as early as 1978 by R. C. Taylor et al. in J. Appl. Phys. 49, 2885 (1978), but the level of interest and activity accelerated only after publication of work on high-energy product bulk materials: melt-spun Fe-Pr and Fe-Nd alloys by J. J. Croat in Appl. Phys. Lett. 37, 1096 (1980), and (Tb, La)-Fe-B alloys by N. C. Koon and B. N. Das in Appl. Phys. Lett. 39, 840 (1981). The main characteristics of these permanent magnets are high coercive force (intrinsic coercivity (.sub.i H.sub.c) of the order of 15 kOe), high remanence (B.sub.r =10 kOe for the oriented materials), and high energy products ((BH).sub.max .gtoreq.40 MGO for the oriented materials).
U.S. Pat. No. 4,496,395 teaches a rare earth-iron permanent magnet consisting essentially of 20-70 atomic % Fe or Fe and Co, the balance being at least one rare earth element such as neodymium. E.P.O. Publ. 0,108,474 teaches an iron-rare earth-boron permanent magnet composition consisting essentially of, in atomic %, 10-50% of at least one rare earth metal with Nd and Pr preferred, 25-9% boron, and 45-85% iron or iron plus cobalt. Each of these references produces its magnets by a rapid solidification process known as "melt spinning" which produces the desired alloy in the form of thin (30-50 micron, max. 200 micron) ribbons (about 1-5 mm wide) which are cooled sufficiently fast so as to produce a very, very fine crystalline structure, but not so fast as to produce an amorphous, i.e. completely glassy, product which in the E.P.O. publication is taught: "cannot be later annealed to achieve magnetic properties comparable to an alloy directly quenched at the optimum rate." (pp 14-15) To form a bulk material from the ribbon, the ribbon is then pulverized into a powder with a roller on a hard surface and the pulverized powder then compacted and magnetized. The pulverizing and compacting steps are not taught as being performed under inert conditions and therefore substantial surface oxidation of the fine powder particles must inherently occur during the production of the bulk crystalline products thicker than 200 microns. No bulk amorphous products can be produced by the procedures disclosed, especially having very low oxygen contents.
E.P.O. Publ. 0,106,948 teaches a permanent magnet composition consisting essentially of, in atomic%, 8-30% of at least one rare earth element, 2-28% boron, not more than 50% cobalt, and the balance iron. The reference states: "It would be practically impossible to obtain practical permanent magnets from [prior art] ribbons or thin films. That is to say, no bulk permanent magnet bodies of any desired shape and size are directly obtainable from the conventional Fe-B-R base melt-quenched ribbons or R-Fe base sputtered thin films." (page 4, 1. 3-8, R=rare earth metal) Therefore, it teaches the preparation of bulk magnet compositions by the steps of (i) casting the desired composition in argon into alloys having a tetragonal system crystal structure, (ii) grinding the alloys to form crystalline grains having sizes of about 1.5 to 50 microns, (iii) orienting the grains in a magnetic field and compacting them in air under pressure, and (iv) sintering the resultant body at elevated temperature in an argon atmosphere. The grinding, which is not performed in an inert atmosphere, inherently produces oxide coatings on the particles formed, thereby substantially increasing the oxygen content of the final body. Since no steps are suggested for removing the oxide surface layer produced, oxygen clearly must be present in the final sintered body in an amount substantially above that produced herein. Moreover, the final body after sintering cannot possibly be amorphous because the sintering step must be performed at such an elevated temperature that any amorphous material would have to be converted to crystalline.
Lee, "Hot-Pressed Neodymiun-Iron-Boron Magnets", Appl. Phy. Lett. (4698) Apr. 15, 1985, pp 790-1, teaches an iron-neodymium-boron permanent magnet powder compact prepared from rapidly quenched alloy ribbons which are then reduced to powder and consolidated. When such powder compacts are bonded by plastics or other materials, it is possible to maintain the initial phase of the starting materials, but the final body has a reduced total metal content, i.e. a density of less than about 85%, and therefore lower magnetic and structural properties. When no binder is used, the subsequent high temperature processing during compacting precludes maintaining the amorphous phase which may have been initially present.
Other references to techniques for fabrication of Fe-Nd-B magnets which include going through a powder stage include melt-spinning, pulverization and consolidation, as taught by J. J. Croat in Appl. Phys. Lett. 37, 1096 (1980); N. C. Koon and B. N. Das in Appl. Phys. Lett. 39, 840 (1981); and J. J. Croat et al. in J. Appl. Phys. 55, 2078 (1984); inert atmosphere powder metallurgy using equilibrium processed alloy as discussed by M. Sagawa et al. in J. Appl. Phys. 55, 2083 (1984); reduction diffusion of Nd-oxide, using the method of Ko-Cheng of the Iron & Steel Research Institute, Peking, China; and activated sintering of constituent elements, as taught by H. H. Stadelmaier et al. in J. Appl. Phys. 56, (1985).
The sequence of rapid solidification processing (RSP) techniques, e.g. melt spinning, twin roller forming, and the like, to form amorphous products which are then pulverized or comminuted into a powder, has led to the discovery that good performance can be achieved with rare-earth/transition metal alloys, for example Fe.sub.77 Nd.sub.15 B.sub.8 and Fe.sub.81 Nd.sub.14 B.sub.5. The raw material costs of such alloys are approximately one third that of Sm-Co alloys and the ingredients are not of a critical nature or an unstable source. Independent research efforts at General Motors Research Laboratories, General Electric Research & Development Center, Naval Research Laboratories, University of Kansas, and Sumitomo Special Metals have converged on the Fe.sub.77 Nd.sub.15 B.sub.8 alloy which has been prepared by the techniques described above. The principal drawback in performance of this alloy seems to be the temperature dependence of remanent induction.
The processing of Fe-Nd-B permanent magnets by techniques which require forming a powder, as discussed above, leaves a great deal to be desired. In particular, the presence of the highly reactive Nd makes prevention of oxidation of the powdery particulate material, which must then be compacted to produce bulk bodies of any substantial size, nearly impossible. Since the presence of oxygen is known to degrade the magnetic performance of magnets as well as their mechanical properties, there is a need for a method of producing bulk magnets in such a manner that the final oxygen content therein is as small as possible, preferably less than about 1,000 ppm.
It is therefore an object of the invention to produce a permanent bulk magnet by using a technique wherein processing parameters are readily controlled such that the microstructure of the material produced can range from crystalline to amorphous and the material generated is in a bulk form so that it does not require subsequent conversion into a powder to generate its desired final shape, i.e. it is directly deposited from a molten alloy of the desired composition. The procedure produces desired permanent bulk magnets while avoiding any significant oxidation of the sensitive constituents.
It is a further object of this invention to provide a permanent bulk magnet comprising readily available, relatively stable and inexpensive constituents.
It is a still further object of the present invention to provide bulk, permanent, isotropic magnets with high intrinsic coercivity, high remanance, and high strength.
It is a still further object of the present invention to produce at least about 90% dense amorphous bulk materials, especially such materials having a thickness of at least 250 microns and, preferably, an oxygen content of less than about 1,000 parts per million.